This invention relates to a process for the manufacture of formaldehyde by the oxidative dehydrogenation of methanol in the presence of a silver catalyst. More particularly this invention relates to a process for the manufacture of formaldehyde by the oxidative dehydrogenation of methanol in the presence of a silver or copper catalyst in which the aqueous formaldehyde product is stripped of methanol and water by distillation at about atmospheric pressure or under reduced pressure and at low reflux in a still which comprises a sufficient number of theoretical plates to allow substantial reversal of the methyl hemiformal reaction and substantial stripping of the methanol.
In the industrial manufacture of formaldehyde from methanol by dehydrogenating oxidation with air over silver or copper catalyst in the presence of steam, generally in a ratio of from 0.1 to 1.8 moles of water per mole of methanol, the formaldehyde is usually washed out of or scrubbed from the reaction gas with water. On absorption of the reaction mixture, the steam produced by the reaction and the steam and methanol left from the starting mixture are condensed. The formaldehyde combines with water to give methylene glycol and higher polyoxymethylene glycols and with residual methanol to give methyl hemiformal and polyoxymethylene glycol monomethyl ethers. The higher polyoxymethylene glycols tend to precipitate from concentrated aqueous formaldehyde solutions as paraform. Hence aqueous formaldehyde has been conveniently used at a concentration of about 30 to about 37% by weight since such solutions are stable over extended periods of time without precipitation of paraformaldehyde and have been conveniently used in the manufacture of phenolic and amino resins.
In more recent times, with the development of improved stabilizers to suppress paraformaldehyde formation, higher concentrations of aqueous formaldehyde have been accepted for the handling of formaldehyde and the manufacture of resins, and have provided energy savings since they improve shipping and handling efficiency and reduce the amount of water to be removed during the resin product preparation. Such concentrated solutions are generally prepared by distilling the aqueous formaldehyde solutions formed in absorbers at substantially atmospheric pressure under conditions which allow most of the unconverted methanol to be removed. The distillation step requires high reflux ratios and considerable energy input.
The present invention is an improved process for the preparation of aqueous formaldehyde solutions in which substantial amounts of methanol and water are removed from the aqueous formaldehyde solution formed in the absorber, by distilling the solution at about atmospheric or under reduced pressure in a still comprising at least about 1.5 theoretical plates. In contrast with the high reflux ratios conventionally used in the removal of methanol by distillation at atmospheric pressure, little or no reflux is used and considerable improvement in energy efficiency of the process is realized. By means of the distillation, a concentrated aqueous formaldehyde solution of low methanol content can be readily obtained and can be used advantageously in the manufacture of phenolic and amino resins. Methanol contents of less than 2 weight percent are readily obtained.
In summary, the invention is a process for the manufacture of an aqueous solution of formaldehyde comprising the steps of:
(a) oxidatively dehydrogenating methanol with air in the presence of a silver or copper catalyst and steam at elevated temperature;
(b) absorbing the reaction product in an absorption train comprising one or more absorption stages in series to form an aqueous formaldehyde solution containing free and combined methanol; and
(c) distilling at low reflux a substantial fraction of the methanol from the aqueous formaldehyde solution in a distillation column comprising at least about 1.5 theoretical plates for methanol distillation, the still pressure at the top of the column being in the range of about 10 to 105 kPa.
Essentially the process is advantageously carried out continuously in the following sequence:
1. a mixture of water and methanol vapors is mixed with air, the mixture is passed over a silver or copper catalyst bed and the methanol is oxidatively dehydrogenated;
2. the reaction product comprising a gaseous mixture of formaldehyde, steam, residual methanol, nitrogen, carbon dioxide and hydrogen is cooled and fed to an absorber comprising a series of stages containing aqueous formaldehyde solution, each stage being equipped with a scrubbing and cooling system. Sufficient water or dilute aqueous formaldehyde solution is added to the final absorber stage to maintain the desired concentration of formaldehyde in the final product, and aqueous formaldehyde solution is passed through the absorber countercurrently to the reaction gas mixture to absorb most of the formaldehyde, water and methanol from the gas mixture. The off-gas from the absorber comprising nitrogen, some carbon monoxide, carbon dioxide and hydrogen, and minor amounts of water, methanol and formaldehyde issues from the top of the absorber and can be burned for its fuel value;
3. aqueous formaldehyde solution is fed from the absorber to a distillation column comprising at least about 1.5 theoretical plates, operated at a pressure in the range of about 10 to about 105 kPa at the top of the column;
4. at least a part of the vapor stream is recycled to the initial reaction mixture comprising water and methanol and the remainder is returned to the top of the still column as reflux, the reflux ratio being about 5 or less;
5. the aqueous formaldehyde solution which is drawn from the bottom of the distillation column constitutes the final formalin product from the process.
In general the absorber can contain any number of absorption or scrubber stages for absorption of the reaction products of the oxidative dehydrogenation of methanol. Conveniently from 2 to 4 absorption stages each equipped with a scrubbing and cooling system may be used. Sufficient water is continuously added to the system to maintain the desired formalin product concentration, and the temperatures and concentrations of the aqueous formaldehyde solutions in the stages are preferably maintained at levels for effecient absorption of formaldehyde and methanol from the reaction gas stream without formation of paraform in the stages. Part of the absorbing stream from at least the first absorption stage is passed to the distillation column. Preferably none of the aqueous formaldehyde solution passed to the distillation column from the first absorption stage is returned directly to the absorber. It is preferably taken off at the bottom of the stripping column as formalin product. Optionally part of the absorbing stream from at least the first two absorption stages can be passed to the still column, the points of entry being intermediate to top and bottom of the still column with the more dilute aqueous formaldehyde solution entering near the top and the more concentrated aqueous formaldehyde solution entering near the bottom while at the same time the more dilute aqueous formaldehyde solution after descent in the distillation column may be partly drawn off as a side stream above the entry point for the more concentrated solution and added to the absorption stream of the prior more concentrated aqueous formaldehyde absorption stage to maintain the concentration of formaldehyde in that absorption stage at a level to avoid formation of paraform at the particular temperature of the stage. Where part of the aqueous formaldehyde solution in an absorption stage other than the first absorption stage is supplied to the distillation column the side stream from the distillation column may provide the only route whereby formaldehyde solution from that absorption stage can flow to the prior absorption stage containing more concentrated aqueous formaldehyde. The distillation column can have as many entering streams as there are absorption stages in the absorber, the streams providing a formaldehyde concentration gradient in the distillation column.
The operation of the distillation column is dependent on several variables including the temperature of the column, the pressure of the column, the reflux ratio, the number of ideal stages or theoretical plates in the column, the residence time of the aqueous formaldehyde solution and the number and the composition of aqueous formaldehyde streams supplied to the column from the absorber.
The temperature and pressure of the column are directly dependent variables. Therefore the temperature required to cause the aqueous formaldehyde solution to boil will be high if the selected column pressure is high and will be low if the column pressure is low. Since the column is operated under steady state conditions at flow rates of liquid and vapor which allow efficient distillation without entrainment of the liquid in the vapor stream, pressure and temperature will increase from top to bottom of the column, the increase being dependent on the number of theoretical plates in the column.
Advantageously the pressure at the top of the column is less than about 50 kPa and is preferably less than about 35 kPa to provide a column temperature at which the volatility of methanol relative to formaldehyde enhances the ratio of methanol to aqueous formaldehyde in the vapor stream emerging from the distillation column and improves the efficiency of methanol stripping. The pressure at the top of the column is advantageously maintained at least at about 10 kPa so that the vapor density is sufficiently high to avoid the need for an excessively wide distillation column to obtain efficient distillation stripping of commercial quantities of aqueous formaldehyde solution. Preferably the pressure at the top of the column is in the range of about 20 to about 35 kPa and the temperature is in the range of about 60.degree. to about 85.degree. C., to allow the vapors to be condensed readily and returned in part as a reflux to the distillation column. The reflux ratio is advantageously about 5 or less and is preferably about 2.5 or less.
In order to obtain significant removal of methanol from the aqueous formaldehyde by means of the distillation column without excessive removal of formaldehyde, the distillation column should comprise at least about 1.5 theoretical plates and preferably about 3 or more theoretical plates. The number of theoretical plates is determined from the following relationship: ##EQU1## where NTP=number of theoretical plates in the distillation column under steady state conditions,
P.sub.t =vapor pressure of methanol in the vapor stream emerging from the top of the distillation column, PA1 P.sub.b =vapor pressure of methanol in the vapor stream at the bottom of the distillation column, PA1 .DELTA.P.sub.t =P.sub.t (e)-P.sub.t PA1 P.sub.t (e)=equilibrium vapor pressure of methanol for the aqueous formaldehyde/methanol solution at the top of the column, at the temperature at the top of the column, PA1 .DELTA.P.sub.b =P.sub.b (e)-P.sub.b PA1 P.sub.b (e)=equilibrium vapor pressure of methanol for the aqueous formaldehyde/methanol solution at the bottom of the column, at the temperature at the bottom of the column.
The equilibrium vapor pressures are determined from standard vapor liquid equilibrium data, for example they may be obtained from data stored in the data base sold by Monsanto under the registered trademark Flowtran.
In aqueous formaldehyde solutions a methanol-methyl hemiformal-polyoxymethylene glycol monomethyl ether equilibrium exists and favors the hemiformal and monoethyl ethers derived therefrom. The equilibrium can be displaced towards methanol by raising the temperature and by dilution of the formaldehyde solution with water. Methanol can be readily removed from dilute aqueous formaldehyde solutions by fractional distillation. However, with concentrated solutions of aqueous formaldehyde which are gaining commercial favor, wherein the formaldehyde concentration is above about 40 weight percent and particularly wherein the formaldehyde concentration is above about 50 weight percent, processes to remove methanol and in particular the distillation process of the present invention to remove methanol at the relatively low temperatures used with the purpose of conserving energy, is more difficult because most of the methanol is chemically combined as hemiformal or monomethyl ethers at these temperatures. Although reversal to methanol occurs progressively with the removal of methanol from the solutions, the rate of reversal is rather low. Efficient removal of methanol in a distillation column comprising conventional packing or tray columns would require an excessively high column. It is therefore advantageous to increase the residence time of the aqueous formaldehyde in the stripping column by any convenient means to allow reversal of the reactions which tie up the methanol. Preferably residence zones are introduced to allow the aqueous formaldehyde solution to reside in the column for at least about 20 minutes. The column then becomes a series of stripping zones separated by residence zones, with the free methanol being generated by reversal of the methanol hemiformal reaction in the residence zones. One way to obtain residence zones is by introduction into the column of a number of chimney trays which are essentially overflow liquid trays with gas chimneys to allow the ascending vapors to pass by the aqueous formaldehyde solution held in the chimney trays. Another way is by means of circulation loops placed at intervals along the distillation column, the loops being equipped with reservoirs of suitable size to isolate the formaldehyde solution from the vapor stream for the desired time. Thus with distillation columns comprising conventional packing or trays such as sieve trays, glass trays, bubblecap trays or valve trays to provide intimate contact between the aqueous formaldehyde solution and the vapor for efficient separation of methanol by the vapor stream, it is advantageous to include resistance zones at intervals in the column to reduce the height of the column required for efficient stripping of methanol. For example a 30 meter distillation column capable of handling about 5 metric tons of formalin product per hour, provides about four theoretical plates for the stripping of methanol determined by means of the relationship set forth above, when it is packed with 21 meters of Pall ring packing divided into 5 zones with each zone separated with a chimney tray of 10 cm. depth, providing a residence time of about 6 minutes in each residence tray. Similarly a 30 meter still column containing 45 sieve trays can provide about four to about seven theoretical plates when 4 residence trays each providing a residence time of about 6 minutes are placed at intervals along the column. Thus by means of the residence zones, theoretical plates of height in the range of about 0.5 to about 10 meters are readily obtained and allow the weights of vapor and liquid passing through the distillation column per unit time to be of about the same order of magnitude. Preferably the weight ratio is in the range of 0.3 to about 1.5.
The vapors which emerge from the distillation column comprising methanol, formaldehyde and water also entrain minor amounts of nitrogen, hydrogen and carbon dioxide carried over from the absorption train. Depending on the initial concentration of methanol in the aqueous formaldehyde solution and the operating conditions of the distillation column, the vapors can be richer in methanol by as much as twenty times the concentration of methanol in the aqueous formaldehyde solution supplied to the column from the absorption train. At the same time some increase in the water concentration is obtained in the vapors compared with the aqueous formaldehyde solution fed to the column. As a result the aqueous formaldehyde solution obtained as product at the still bottom is enhanced in formaldehyde concentration and is considerably reduced in methanol concentration.
Advantageously, the vapors emerging from the top of the distillation column can be fed directly to the vaporized reaction mixture of methanol and water with suitable adjustment to maintain the desired ratio of methanol and water in the reaction mixture. Such direct recycle is especially advantageous when the concentration of methanol in the aqueous formaldehyde solution fed from the absorption train to the distillation column is already quite low, for example less than about 4 weight percent. However, when the concentration of methanol in the aqueous formaldehyde solution fed from the absorption train to the distillation column is greater than about 4 weight percent, recycle of all the vapors emerging from the distillation column to the converter may be undesirable because such recycle would generate a rather high concentration of formaldehyde in the reaction mixture and would tend to harm the reactor time/space relationship and increase the thermal energy used in distillate recycle vaporization. Optionally therefore the vapors issuing from the top of the distillation column may be passed to a reflux condensor to condense an aqueous formaldehyde solution which is rich in methanol. Part of the condensate can be returned to the distillation column to increase the efficiency of the stripping process and part can be vaporized and recycled to the converter. Advantageously the ratio of refluxed to recycled condensate is selected so that the mole ratio of recycled formaldehyde to total methanol fed to the reactor is less than about 0.035.
Because a major portion of the methanol present in the aqueous formaldehyde solution fed to the distillation column can be advantageously removed by the stripping action, the aqueous formaldehyde-methanol solution recycled to the reactor is characterized by a methanol to formaldehyde mol ratio of at least about 0.6, and a ratio of at least about 1.0 can be readily achieved. This mol ratio is generally at least about 10 times higher than the ratio of the solution fed to the column from the first absorption stage of the absorber.
In comparison with a rectifying distillation column operated at 2/3 to one atmosphere pressure for removal of almost pure methanol from the aqueous formaldehyde solution produced in the absorber, the stripping process of the present invention can reduce the energy requirement for methanol removal by about 50 percent or more without significant sacrifice in separating efficiency.
Since the distillation column, and particularly the lower stages of the column, is operated in the present invention at relatively low temperatures in comparison with a usual rectifying distillation column, and since the reflux returned to the distillation column is a minor fraction of the total amount of aqueous formaldehyde solution in the column, heat required to maintain the column at the operating temperature can be readily supplied by the absorption solution from the first stage of the absorber optionally supplemented by a small reboiler unit at the bottom of the column.